Conventional methods for developing an electrostatic latent image formed on an electrostatic charge image carrier such as an electrophotographic photoreceptor or an electrostatic recording body are roughly classified into two types; one being a type of methods using a liquid developer wherein fine toner particles are dispersed in an electrically insulating liquid (wet-process developing method), and the other being a type of methods using toner particles wherein a colorant or a magnetizable powder is dispersed in a binder resin (dry-process developing method). As the dry-process developing method, there are known a method of using a two-component developer comprising carrier particles and a toner and a method of using a single-component developer consisting of a toner (generally a magnetic toner).
Toners for use in development of electrostatic charge images according to the dry-process developing method are commonly produced by using a styrene-based resin or a polyester-based resin as a binder resin, kneading a colorant such as a dye or a pigment into the resin and, after cooling, subjecting the resultant mixture to a pulverizing step and a classifying step. The toner for developing electrostatic charge images commonly has an average particle size of about 1 to about 30 μm. With magnetic toners, a magnetic powder of magnetite or the like is further used. As the carrier particles to be used in the two-component developers, glass beads, iron powder or magnetite powder coated, if necessary, with a hydrophobic resin are used.
The toners for developing electrostatic charge images as described above are required to maintain positive or negative charge depending upon the polarity of the electrostatic latent image to be developed. In order to maintain electric charge on the toner for developing the electrostatic charge image, triboelectric properties of toner components such as a binder resin may be utilized. However, triboelectric charge alone is generally insufficient in electrostatic charge amount and is unstable in polarity, and hence images obtained by development utilizing only triboelectric charge tend to be fogged and blurred. Therefore, in order to impart desirable charging properties to a toner, it is common to add a substance called a charge controlling agent to the toner.
Typical examples of conventional charge controlling agents include those which impart positively charging properties to a toner such as basic dyes (e.g., nigrosine dyes (Japanese Examined Patent Publication No. S48-25669) and triarylmethane dyes), quaternary ammonium salts (Japanese Unexamined Patent Publication No. S57-119364), organic tin oxides (Japanese Examined Patent Publication No. S57-29704) and electron donative substances such as amino group-containing polymers; and those which impart negatively charging properties to a toner exemplified by metal-containing dyes such as metal complexes of monoazo dyes and chromium-containing organic dyes (e.g., Copper Phthalocyanine Green, chromium-containing monoazo dyes, etc.).
In recent years, as photoreceptors for electrophotographic copiers or printers, organic photoreceptors have popularly been used and, as a developer for copiers or the like using the organic photoreceptors, positively charging toners have been required. However, many of positive charge-imparting charge controlling agents to be used for producing such positively charging toners are hydrophilic and, under the condition of a high humidity, they tend to cause reduction in charge quantity, whereas many of those which do not cause reduction in charge quantity under highly humid conditions cause the problem that there results an unnecessarily high charge quantity under the condition of a low humidity. For example, triarylmethane dyes tend to cause reduction in charge quantity under the conditions of a high temperature and a high humidity, though they show excellent positively charging properties under the conditions of an ordinary temperature and an ordinary humidity and, with respect to particularly selected toner components, they might generate non-uniform charge under the conditions of a low temperature and a low humidity. Quaternary ammonium salts have a low charge-imparting ability, and there might result a decrease in image density with an increase in number of produced copies under the conditions of a high temperature and a high humidity.
On the other hand, nigrosine dyes have high charge-imparting properties and are therefore comparatively excellent charge controlling agents, but they show such a poor dispersibility in a toner resin that they are difficult to be uniformly dispersed. In addition, the charging property and thermal property of a toner largely vary depending upon the process for their production, and it is difficult to obtain charge controlling agents fully satisfying conditions required for charge controlling agents—for example, there results a decrease in image density, fogging is liable to occur, or a problem of dusting of toner particles inside of a copier arises. In order to dissolve these problems, there are proposed techniques of lake-formation and acid-modification of nigrosine dyes (Japanese Unexamined Patent Publication No. H3-13186). However conducting such treating, there might result a decrease of dispersibility of charge controlling agent in a toner or fluidity of a toner.
Toners using the nigrosine dye or the nigrosine pigment involve another problem that, when a large amount of the toner is replenished at once to a developer vessel upon replenishing the toner, there results a temporary reduction in image density and the image density does not return to the level before replenishment of the toner until several tens to several hundreds of copies are produced. Further, the toners involve the problem that they suffer deterioration of characteristic properties due to heat stress to be applied thereto during storage.
As is described above, the nigrosine dyes and the nigrosine pigments involve many problems, though they are comparatively excellent charge controlling agents. Thus, there have been required a charge controlling agent which solve these problems, which provides an enough high charge quantity, which shows less dependence upon temperature and humidity upon development, which suffers less deterioration in toner characteristic due to heat stress to be applied thereto before use, which shows a good dispersibility in a binder resin, shows excellent developing properties, and which shows excellent lasting properties, and a positively charging toner containing the same.